polymer c€¦ · interests include liquid crystalline polymers and conjugated...
TRANSCRIPT
Dynamic Article LinksC<PolymerChemistry
Cite this: Polym. Chem., 2011, 2, 2707
www.rsc.org/polymers REVIEW
Publ
ishe
d on
24
Aug
ust 2
011.
Dow
nloa
ded
on 2
5/04
/201
4 02
:09:
05.
View Article Online / Journal Homepage / Table of Contents for this issue
Conjugated polymer nanostructures for organic solar cell applications
Jiun-Tai Chen* and Chain-Shu Hsu*
Received 16th June 2011, Accepted 17th July 2011
DOI: 10.1039/c1py00275a
Recently, there has been tremendous progress in the development of polymer-based organic solar cells.
Polymer-based solar cells have attracted a great deal of attention because they have the potential to be
efficient, inexpensive, and solution processable. New materials, nanostructures, device designs, and
processing methods have been developed to achieve high device efficiencies. This review focuses on the
fabrication techniques of conjugated polymer nanostructures and their applications for organic solar
cells. We will first introduce the fundamental knowledge of organic solar cells and emphasize the
importance of nanostructures. Then we will discuss different strategies for fabricating conjugated
polymer nanostructures, including topics such as polymer nanowires, nanoparticles, block copolymers,
layer-by-layer deposition, nanoimprint lithography, template methods, nanoelectrodes, and porous
inorganic materials. The effects of the nanostructures on the device performance will also be presented.
Efficiencies higher than 10% are expected for polymer-based solar cells by using new materials and
techniques.
Department of Applied Chemistry, National Chiao Tung University, 1001Ta Hsueh Road, Hsin-Chu, 30049, Taiwan. E-mail: [email protected]; [email protected]; Fax: +886-3513-1523; Tel: +886-3513-1523
Jiun-Tai Chen
Jiun-Tai Chen received his B.S.
degree in 1999 and M.S. degree
in 2001 from the Department of
Applied Chemistry at National
Chiao Tung University. He
joined Prof. Thomas Russell’s
group in 2003 and completed his
Ph.D. in 2008 at the University
of Massachusetts, Amherst in
Polymer Science and Engi-
neering, where his thesis work
focused on template-based
nanomaterials. He then joined
the Center for Nano- and
Molecular Science and Tech-
nology at the University of
Texas at Austin with Prof. Paul F. Barbara as a postdoctoral
fellow, where he worked on electrogenerated chemiluminescence of
conjugated polymers. In the summer of 2010, he joined the
Department of Applied Chemistry at National Chiao Tung
University as an assistant professor. His research interests include
the fabrication and characterization of polymer nanomaterials for
optoelectronic applications.
Chain-Shu Hsu
Chain-Shu Hsu received his
Ph.D. degree from Case
Western Reserve University in
1987 and conducted post-
doctoral work at the National
Tsing Hua University in Tai-
wan. He joined the Department
of Applied Chemistry of the
National Chiao Tung Univer-
sity, Taiwan in 1988 as an
associate professor and was
promoted to full professor in
1991. Currently he is serving as
a vice president and chair
professor of the National Chiao
Tung University. His research
interests include liquid crystalline polymers and conjugated poly-
mers, polymer light-emitting diodes, and organic solar cells. He has
published more than 200 research papers and 20 patents. He is
currently on the international advisory board of Polymer and
editorial boards of the Journal of Polymer Science, Polymer
Chemistry, and the Journal of Polymer Research. He received the
Excellent Research Award of the National Science Council, Tai-
wan, in 1994, the Franco-Taiwan Scientific Award for nano-
materials in 2006, Teco and Hou Chin Tui Awards in 2007, and an
Academic Award of the Ministry of Education, Taiwan, in 2008.
This journal is ª The Royal Society of Chemistry 2011 Polym. Chem., 2011, 2, 2707–2722 | 2707
Publ
ishe
d on
24
Aug
ust 2
011.
Dow
nloa
ded
on 2
5/04
/201
4 02
:09:
05.
View Article Online
1 Introduction
1.1 Organic solar cells
In recent years, energy-related issues have received considerable
attention concerning the rising costs of fossils and growing global
greenhouse gas.1 There is an urgent need to develop clean and
renewable energy technologies. The largest potential source of
renewable energy is the solar energy incident on the Earth’s
surface.2 Solar cells are devices which convert solar energy
directly into electricity, and the most common material used for
solar cells is silicon.3 Although silicon-based solar cells exhibit
some of the highest power conversion efficiencies, they remain
expensive because of the intensive processing techniques and the
high cost of purified silicon.4 In this context, organic molecules
are alternative candidates for solar cells because of their low cost
and high processability.5,6
The most widely studied organic solar cells are polymer-based
solar cells using conjugated polymers. Most conjugated polymers
have high absorption coefficient and high percentage of absorbed
photons that can produce an excited state (>90%).7 There are
other advantages for using the polymer-based organic solar cells.
First, the photo and electronic properties of the conjugated
polymers can be fine-tuned by changing the chemical structures
through advances in organic chemistry.8,9 Second, simple coating
or printing processes can be used which will reduce the cost of the
fabrication process.10 Third, the mechanical flexibility allows the
development of flexible devices.11 In addition to organic solar
cells, conjugated polymers have been used in other types of
optoelectronic devices such as organic field-effect transistors
(OFETs) or organic light-emitting diodes (OLEDs).12,13
Although significant progress has been made in polymer-based
solar cells, the maximum power conversion efficiencies (PCEs)
are still not sufficient to be marketable. Also, the commerciali-
zation of the polymer-based solar cells is limited by the lifetimes
of the devices which are affected by the water and oxygen in the
atmosphere. Therefore, more research effort will need to be
devoted towards the commercialization of polymer-based solar
cells.14
1.2 Working principles of organic solar cells
The typical structure of an organic solar cell is shown in Fig. 1. A
hole transport layer, poly(3,4-ethylenedioxythiophene):poly
(styrenesulfonate) (PEDOT:PSS), is spin-coated on top of the
Fig. 1 Typical structure of an organic solar cell. PEDOT:PSS is spin-
coated on top of the anode as a hole transport layer. The active layer is
sandwiched between the cathode and the hole transport layer.
2708 | Polym. Chem., 2011, 2, 2707–2722
anode. The active layer comprising the donor and the acceptor is
sandwiched between the cathode and the hole transport layer.
The process for organic solar cells to convert sunlight into elec-
tricity is described as follows: The light-absorbing material with
a bandgap in the visible region absorbs photons that excite the
electrons from the ground state to the excited state, and bound
electron-hole pairs (excitons) are created. The excitons diffuse to
the donor–acceptor interface where excitons dissociate into free
charge carriers after overcoming the binding energies. The free
charge carriers transport to the respective electrodes under the
internal electric fields, resulting in the generation of
photocurrent.
Power conversion efficiency (PCE) is used to evaluate the
performance of polymer solar cells.5 The PCE of an organic solar
cell is determined by the following equation:
PCE ¼ (FF � Jsc � Voc)/Pin (1)
where PCE is the power conversion efficiency, FF is the fill
factor, Voc is the open-circuit voltage, Jsc is the short-circuit
current, and Pin is the power density of the incident light. The
solar cells are usually tested under Air Mass (AM) 1.5G condi-
tions, 100 mW cm�2. These conditions are experienced when the
sun is at an angle of about 48� and are considered to best
represent the Sun’s spectrum on the Earth’s surface.15 Fill factor
is the ratio of the actual power limit to the theoretical power limit
of a solar cell, which can be calculated from the division of the
largest power output (Pmax) by the product of Jsc and Voc, as
shown in Fig. 2. Open-circuit voltage (Voc) is the maximum
possible voltage across a solar cell. The value ofVoc is close to the
energy difference between the highest occupied molecular orbital
(HOMO) of the electron donor and the lowest unoccupied
molecular orbital (LUMO) of the electron acceptor (see Fig. 3).16
The short-circuit current (Jsc) is the current through the solar cell
when the voltage across the solar cell is zero.
One of the most promising candidates for polymer organic
solar cells is a blend of regioregular poly(3-hexylthiophene) (rr-
P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)
(Fig. 4).17 Although the bandgap is only 2.0 eV, P3HT possesses
high hole mobility (up to 0.1 cm2V�1s�1) and great self-organi-
zation capability.18,19 The mobility of P3HT is highly dependent
Fig. 2 Current–voltage (I–V) characteristics of an organic solar cell. Voc
is the open-circuit voltage and Jsc is the short-circuit current. The points
where the product of current and voltage is maximized determine the
largest power output (Pmax). The fill factor (FF) is obtained from the
division of Pmax by the product of Jsc and Voc.
This journal is ª The Royal Society of Chemistry 2011
Fig. 3 Energy level diagram of an organic solar cell with a donor–
acceptor interface. The open-circuit voltage (Voc) is close to the energy
difference between the highest occupied molecular orbital (HOMO) of
the donor and the lowest unoccupied molecular orbital (LUMO) of the
acceptor.
Fig. 4 Chemical structures of poly(3-hexylthiophene) (P3HT) and [6,6]-
phenyl-C61-butyric acid methyl ester (PCBM).
Fig. 5 Main research directions to improve the device performance of
organic solar cells including materials, devices, morphology, and
nanostructures.
Publ
ishe
d on
24
Aug
ust 2
011.
Dow
nloa
ded
on 2
5/04
/201
4 02
:09:
05.
View Article Online
on the crystallinity, the orientation of the polymer chains, and
the molecular weight. PCBM has a high electron mobility and is
soluble in common organic solvents. The combination of P3HT
and PCBM dominates the current research in polymer solar cells.
They also serve as good model materials to examine different
effects on the device performance. In this review, many examples
of solar cells based on conjugated polymer nanostructures are
related to the fabrication of P3HT nanostructures followed by
the deposition of PCBM. For example, P3HT nanorods can be
made by a nanoporous template and function as the donor
materials. Then PCBM can be deposited on the P3HT nanorods
to form an ordered heterojunction.
1.3 Strategies to improve the device performance
After introducing the fundamental knowledge of organic solar
cells, we will discuss some common strategies to improve the
device performance of organic solar cells. As shown in Fig. 5,
these strategies involve designing and synthesizing newmaterials,
changing device structures, controlling morphology, and making
nanostructures.
Although the Sun provides a vast amount of energy, only
a small portion of the incident sunlight is absorbed because of the
large bandgap of organic materials. The bandgap of typical
conjugated polymers ranges between 2 and 3.5 eV, which limits
the possible absorption of solar energy. For a material with
This journal is ª The Royal Society of Chemistry 2011
a band gap of 1.1 eV (1100 nm), 77% of the incident solar energy
on the Earth’s surface can be absorbed.5 Therefore, it is necessary
to design and synthesize new low bandgap conjugated polymers
that can absorb more of the solar energy. It also need to be noted
that the device performance can be affected by not only the
bandgap, but also the position of the HOMO and LUMO levels
of the conjugated polymers which can limit the Voc of the
device.16 The absorption spectrum and energy levels of conju-
gated polymers can be tuned by functionalization. Many
synthetic efforts have been made to develop new low bandgap
polymers.20,21
Towards the commercialization of organic solar cells, several
issues with the standard organic solar cells have to be considered.
First, the low work-function metal electrodes, such as calcium
and lithium, are unstable under ambient conditions.22 Second,
the hole transport material, PEDOT:PSS, has been shown to
react with the ITO electrode, resulting in the degradation of the
devices.23 The acidic PEDOT:PSS layer is also detrimental to the
active layer. To resolve these issues, inverted device structures
have been developed which allows the use of more stable high
work-function metals.24 In the inverted structures, electrons and
holes exit the device in opposite directions, comparing with the
normal device structures. ITO serves as the cathode and a more
stable, high-work-function metal is used as the anode. The
stability of the inverted device under ambient conditions is
dramatically improved, compared with the normal device.25
The morphology control of the active layer in solar cells is of
great importance in improving the device efficiencies. Since
exciton dissociation occurs at the interface of the donor and
acceptor materials, a large interfacial area should allow
maximum exciton dissociation.26 It has been shown that post-
treatments such as thermal annealing above the glass transition
temperature (Tg) of the active material are crucial for the device
performance, although low band-gap polymers often show
degraded device performance after thermal annealing.27,28 By
annealing, the active materials such as the P3HT chains can
organize and self-assemble into a more regular, crystalline state,
resulting in higher charge mobility. Acceptor materials such as
PCBM will also diffuse and aggregate to form larger domains.
The device performance is improved by the maximized donor–
acceptor interfacial area and the higher degree of crystallinity.
But the optimized morphology and phase segregation of the
Polym. Chem., 2011, 2, 2707–2722 | 2709
Publ
ishe
d on
24
Aug
ust 2
011.
Dow
nloa
ded
on 2
5/04
/201
4 02
:09:
05.
View Article Online
active materials are at intermediate states that will form more
equilibrium morphologies upon further annealing.29 With longer
annealing time, the donor–acceptor interfacial area decreases
owing to larger size phase separation, resulting in lower effi-
ciencies. Many efforts have been made to maintain the optimized
morphology of the donor–acceptor heterojunction. Photo-
crosslinkable P3HT copolymers or fullerene derivatives, for
example, are used to stabilize the bulk heterojuncitons.30–33 The
morphology of the heterojunction can also be preserved by using
a block copolymer containing oligothiophene and fullerene side
groups as a compatibilizer.26
Solar cells based on polymer heterojunction are considered to
be better than single component polymer solar cells. The inter-
facial area between the donor and acceptor materials can be
increased and more exciton dissociation can occur to generate
higher photocurrent. The concept of heterojunction was first
introduced by Tang: bilayer heterojunction structures can be
used for creating efficient charge separation.34 This concept was
then applied to polymer-based solar cells and the efficiency was
dramatically increased, because of the large donor–acceptor
interfaces.35 Bulk heterojunction polymer solar cells are usually
based on two components including an electron-donating
material (donor) and an electron-accepting material (acceptor).
Disordered structures on the nanoscale will result in poor device
efficiencies due to exciton recombination and poor mobility.36 It
is necessary to control the morphology of the active materials on
the nanoscale. There are several possible morphologies for the
donor–acceptor heterojunction. The simplest case is the bilayer
structures, where the electron-donor is first deposited on the
anode, followed by the deposition of the electron-acceptor (see
Fig. 6a).37 But the bilayer devices only allow excitons to disso-
ciate near the donor–acceptor interface, resulting in low device
efficiencies.38 In order to improve the solar cell efficiencies, it is
critical to control the donor–acceptor interface in order to
optimize charge separation and charge migration to the elec-
trodes. The most common way to make bulk heterojunction
organic solar cells is by blending donor and acceptor materials.
The interfacial area between donor and acceptor significantly
increase compared with the double layer devices, resulting in
improved device efficiencies (see Fig. 6).
In order to achieve high device performance, the domain sizes
of the donor and the acceptor need to be optimized. The optimal
domain size is related to the diffusion length of excitons. Exciton
diffusion length is the average distances the excitons travel before
recombination, which depends on the lifetime and the diffusion
coefficient of the excitons. The diffusion length of excitons in the
polymer-based organic solar cells is usually around 5–10 nm.
Therefore, the ideal size of the nanostructures should be close or
equal to the diffusion length. A larger interface is produced by
Fig. 6 Three possible donor–acceptor morphologies of organic solar
cells. (a) Double layer morphology. (b) Phase-separated donor–acceptor
blend morphology. (c) Ideal heterojunction morphology.
2710 | Polym. Chem., 2011, 2, 2707–2722
reducing the domain sizes, allowing more exciton dissociation
while reducing the recombination of excitons. Fig. 6c represents
the ideal heterojunction morphology of organic solar cells, where
the donor and the acceptor domains are aligned normal to the
electrode surfaces.39 Also, there should be a continuous donor
film in contact with the anode and a continuous acceptor film in
contact with the cathode. The thickness of the heterojunction
also needs to be optimized. The light absorption depends on not
only the bandgap of the materials, but also the thickness of the
absorbing materials. The absorption coefficients of conjugated
polymers are relatively high (�105 cm�1), and the film thickness
of 100–200 nm should allow efficient absorption.5,36 Higher
thickness will increase chances of recombination of the excitons,
even though more photons might be absorbed. The crystallinity
and charge mobilities can also be changed by the confinement
effect at the nanoscale. For example, the hole mobility in
regioregular P3HT was found to enhance by a factor of 20 when
the polymers were infiltrated into straight nanopores of an
anodic alumina template.40 In addition to the diffusion length of
excitons, the optimized domain sizes also depend on the packing
of molecules, which affects the charge transport in devices,
concerning the p–p interaction between conjugated polymer
chains.
There are some excellent review articles on conjugated poly-
mers and polymer-based solar cells.5,6,8,11,17,24,41,42 This review
aims to highlight different fabrication techniques for polymer
nanostructures for the application of organic solar cells. As
shown in Fig. 7, these methods are divided into subjects including
polymer nanowires, nanoparticles, block copolymers, layer-by-
layer deposition, nanoimprint lithography, template methods,
nanoelectrodes, and porous inorganic materials. Here we mainly
focus on the nanostructures of the donor materials such as
P3HT. In order to make heterojunction solar cells, the acceptor
materials are usually deposited on the nanostructured donor
materials. A typical example is to make P3HT nanorods using
templates or nanoimprint lithography, then the PCBM can be
deposited on the P3HT nanorods for constructing the donor–
acceptor heterojunction. Although here we only focus on the
discussion of donor nanostructures, these concepts can be
applied to make the acceptor nanostructures. For example, C60,
TiO2 or ZnO nanorods can be generated by porous templates,
followed by the spin-coating of the P3HT.43–46
Fig. 7 Approaches to fabricate conjugated polymer nanostructures for
applications in organic solar cells.
This journal is ª The Royal Society of Chemistry 2011
Publ
ishe
d on
24
Aug
ust 2
011.
Dow
nloa
ded
on 2
5/04
/201
4 02
:09:
05.
View Article Online
2 Strategies for fabricating polymer nanostructures
This part summarizes some common strategies and techniques
for making polymer nanostructures for the application of
organic solar cells. The device performance based on different
nanostructures will also be presented.
Fig. 8 Schematic illustration showing an example of using the whisker
method to make P3HT nanowires. P3HT polymers are dissolved in p-
xylene and heated to 80 �C. The solution is later cooled down to room
temperature and P3HT nanowires are formed.
2.1 Polymer nanowires
One of the most common conjugated polymer nanostructures
used in organic solar cells is conjugated polymer nanowires, for
they provide percolation pathways for both electrons and holes,
resulting in higher device efficiency. Nanowires are sometimes
called nanocylinders, nanofibers, or nanowhiskers, meaning one-
dimensional nanomaterials with high aspect ratios. The advan-
tages of using the polymer nanowires approach for solar cell
devices include (a) the morphology can be controlled; (b) the
widths and lengths of polymer nanowires are matched to the
exciton diffusion lengths; (c) the interfacial area between donor
and acceptor is large; (d) an electrically bicontinuous
morphology can be obtained; (e) high absorption coefficient and
high carrier mobilities can be achieved; (f) devices on plastic
substrates and devices with large areas can be easily produced;
(g) the difficulties of blend phase-separation phenomena can be
avoided.47,48
Various techniques have been applied to prepare conjugated
polymer nanowires. It has been found that the polymer nano-
wires can be simply observed by thermal annealing.28 For
example, P3HT nanowires were observed after annealing the
P3HT/PCBM mixture at 120 �C for 60 min.28 The annealing
process increases the crystallinity of P3HT and enhances the
demixing between P3HT and PCBM. Another simple method to
generate P3HT nanowires was studied by Sun et al.49 They found
that P3HT nanowires and CdSe nanorods can be obtained by
careful choice of the solvent used for spin-coating. 1,2,4-Tri-
chlorobenzene (TCB), which has a high boiling point, was used
as the solvent for P3HT and a fibrillar morphology was obtained.
The power efficiency of the solar cell devices based on the
composites of P3HT nanowires and CdSe nanorods was
improved to 2.6% (AM 1.5G), compared with 1.8% where
chloroform was used as the solvent and P3HT nanowires were
not formed.49
In addition to the previously mentioned methods, the two
most common ways to make conjugated polymer nanowires for
the applications in organic solar cells are the whisker method and
the mixed-solvent method.
2.1.1 The whisker method. The whisker method was first
developed by Ihn et al.50 They reported that poly(3-alkylth-
iophene)s may readily crystallize from dilute solutions in rela-
tively poor solvents in the form of ribbon-shaped whiskers. The
formation of whiskers is dependent on the solvent quality,
temperature, and the alkyl side-chain length. The widths of the
whiskers are about 15 nm and their lengths often exceed tens of
microns, so very high aspect ratios are observed for the whiskers.
The polymer chains made by the whisker method were found to
pack with their backbones normal to the whisker direction.50
Fig. 8 shows an example using the whisker method to make
P3HT nanowires. P3HT polymers are dissolved in p-xylene and
This journal is ª The Royal Society of Chemistry 2011
heated to 80 �C. The solution is later cooled down to room
temperature and P3HT nanowires are formed.
To address the issue of what solvents are most suitable for
nanofiber formation using the whisker method, Oosterbaan et al.
performed systematic studies of the fiber formation of regiore-
gular poly(3-alkylthiophene)s (P3ATs) with alkyl chain lengths
between 3 and 9 carbon atoms in several solvents.51 For the
aliphatic and (chlorinated) aromatic hydrocarbon solvents, the
refractive index of the solvent was used to predict the feasibility
of a particular solvent for the fiber formation. The effect of poly
(3-alkylthiophene) (P3AT) crystallinity on the energy of the
intermolecular charge-transfer state (ECT) and open-circuit
voltage (Voc) in P3AT nanofibers:PCBM solar cells was also
investigated.52 The P3AT crystallinity can be varied by control-
ling the temperature. The ECT was found to increase slightly with
increasing side-chain length and the Voc followed the same trend
as ECT because of the morphological changes.52
Using the whisker method, P3HT nanofibers are most studied
compared with other P3AT nanofibers. Berson et al. presented
a new fabrication procedure to produce highly concentrated
solutions of P3HT nanofibers in p-xylene.53 The concentration
range of up to 2% allowed the deposition of thick films and the
obtained solutions were stable for several weeks. By mixing these
nanofibers with an electron acceptor such as PCBM in solution,
a highly efficient active layer for organic solar cells with a PCE of
up to 3.6% (AM 1.5G, 100mW cm�2) was obtained without any
thermal post-treatment.53 The maximum PCE was achieved with
the optimum composition of 75 wt% nanofibers and 25 wt%
disorganized P3HT. It was proposed that the fraction of disor-
ganized P3HT is probably necessary to fill the gaps in the
nanostructures and intimate contact between the donor nano-
fibers and the acceptor domains can be ensured.53 The device
efficiency can also be improved by controlling the fiber content of
the casting solution. Bertho et al. demonstrated that the fiber
content of the P3HT-nanofiber:PCBM casting solution can be
easily controlled by changing the solution temperature.54 At
a solution temperature 45 �C, a 42% fiber content of the casting
solution was found and an optimal PCE of 3.2% was achieved,
which was linked to the morphology of the active layer.54
For the P3HT nanowires/PCBM system, Kim et al. tried to
optimize solar cells based on the P3HT nanowires via solution
crystallization in DCM.55 They studied the performances of the
solar cell devices based on the P3HT nanowires/PCBM
composites as a function of solution ageing time. The PCE of
3.23% was achieved for the devices coated with a 60 h aged P3HT
nanowires/PCBM blend solution. The ageing process was
thought to increase both light absorption and charge balance.
Polym. Chem., 2011, 2, 2707–2722 | 2711
Publ
ishe
d on
24
Aug
ust 2
011.
Dow
nloa
ded
on 2
5/04
/201
4 02
:09:
05.
View Article Online
When a pure donor phase layer was inserted between the ITO/
PEDOT:PSS and P3HT nanowires/PCBM layers, an even higher
PCE of 3.94% was achieved.55
For solar cells based on the P3HT nanowires, other electron
acceptors other than PCBM have also been used. Salim et al.
reported the first application of the preassembled P3HT nano-
wires with poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT)
in all-polymer solar cells.56 The role of the polymer nanowires on
the morphology of the polymer blends was investigated.
Compared with as-cast blends, an enhancement in the short-
circuit current (Jsc) by a factor of 10 was achieved when P3HT
nanowires were added into the blends with polyfluorene copol-
ymers. A higher PCE was achieved for the solar cells comprising
the nanowire blends, even compared with the thermally annealed
ones. The enhanced device performance was attributed to the
enhanced optical absorption and charge transport originating
from the high crystallinity of the polymer nanowires.56
Inorganic nanorods have also been used in the solar cells based
on P3AT nanowires. Jiu et al. demonstrated that the use of
preformed poly(3-alkylthiophene) nanowires in hybrid solar cells
with CdSe nanorods achieved a PCE of 1%.57 The device
performance from nanowires prepared from poly(3-butylth-
iophene) (P3BT) was better than that from poly(3-hexylth-
iophene) (P3HT). The CdSe nanorods were found to be dispersed
within the interpenetrated 3-D network of interconnected poly-
mer nanowires, which improved electron and hole transport in
the hybrid film. The maximum PCE of 1.01% (AM 1.5G,
100 mW cm�2) was achieved with the optimum composition of
25 wt% P3BT nanowires and 75 wt% CdSe nanorods.57
Other P3AT nanowires-based solar cells other than P3HT
have also been studied. By using poly(3-butylthiophene) nano-
wires (P3BT-NW) as the donor and PCBM as the acceptor, Xin
et al. reported that solar cells with 3.0% power conversion effi-
ciency (AM 1.5G, 100 mW cm�2, 10 mm2 device area) was ach-
ieved.47 The performance achieved was 1 order of magnitude
higher than that from thermally induced phase-separated P3BT:
PCBM blend. In their approach, P3BT-NW/PCBM nano-
composites exhibited an electrically bicontinuous morphology
without going through the path of blend phase-separation
phenomena. The results indicated that the P3BT NWs constitute
an ideal donor component for enhanced exciton diffusion and
charge transport in solar cell devices.47 Xin et al. also fabricated
bulk heterojunction solar cells based on blends of regioregular
poly(3-butylthiophene) (P3BT) nanowires and phenyl-C61-
butyric acid methyl ester (PCBM) using in situ self-assembly of
P3BT nanowires.48 The approach of in situ self-assembly has the
advantages of simplifying and combining the previously separate
process of preparing P3BT NWs and blending with PCBM into
a single process. The P3BT NWs were found to self-assemble to
an interconnected network in the presence of the PCBM. The
device performance was found to depend strongly on the blend
composition. A maximum PCE of 2.52% was achieved at a blend
ratio of 1 : 0.5 (wt : wt) P3BT : PCBM.48
Although a power conversion efficiency (PCE) of 3.0% is
achieved from the P3BT-NW/fullerene composite,47 the device
structure was not optimized. Xin et al. systematically varied
the morphology of P3BT-NW/fullerene composites by using
a combination of thermal and solvent annealing.58 The
photovoltaic performance was found to vary with the induced
2712 | Polym. Chem., 2011, 2, 2707–2722
structural variations. In unannealed devices, fullerene was
found to disperse homogeneously in the P3BT-NW matrix and
poor photovoltaic performances was obtained. The aggrega-
tion of fullerene in the interstitial spaces of the nanowire
network was induced by thermal annealing and improved
photovoltaic performance was obtained. The authors sug-
gested that the best device performance can be achieved when
the ideal interpenetrating network of nanowires and fullerene
is maintained while avoiding the device bridging of the poly-
mer nanowires.58
Of all the regioregular poly(alkylthiophene) nanowires,
regioregular poly(3-pentylthiophene) (P3PT), which has odd-
numbered alkyl side chains (CnH2n+1, n ¼ 5), is less studied. Wu
et al. first reported the fabrication of regioregular poly(3-pen-
tylthiophene) (P3PT) nanowires and their applications in solar
cells.59 The P3PT nanowires with a width of 16–17 nm and aspect
ratios of 70–465 were assembled from dichlorobenzene solution.
The power conversion efficiency (AM 1.5G, 100 mW cm�2) of
bulk heterojunction solar cells based on P3PT nanowires/
PC71BM nanocomposites was 3.33%, while the power conversion
efficiency of bulk heterojunction solar cells based on P3PT/
fullerene (PC71BM) blend thin films was 3.70%.59
Block copolymer nanowires have also been fabricated using
the whisker method. Ren et al. studied solution-phase self-
assembled nanowires from diblock copolymer semiconductors,
poly(3-butylthiophene)-block-poly(3-octylthiophene).60 For the
copolymer nanowires, the authors tried to control the aspect
ratio of solution phase assembled nanowires and studied the
effects of the aspect ratio on their properties and device perfor-
mances. The aspect ratio of the diblock copolymer nanowires
was controlled by the copolymer composition. The vertical
charge transport of the nanowires/fullerene thin films was found
to be independent of aspect ratio of the nanowires, indicating
a parallel orientation of the nanowires to the underlying
substrate.60 But the power conversion efficiency of the solar cells
based on nanowires/PC71BM nanocomposites was found to be
dependent on the aspect ratio of the nanowires. A power
conversion efficiency of 3.4% was achieved when the highest
average aspect ratio of 260 was used. The improvement of device
performance with the aspect ratio of nanowires was attributed to
the increased exciton and charge photogeneration and collection
in the solar cells.60
2.1.2 The mixed-solvent method. The second common ways
to make conjugated polymer nanowires for the application of
solar cells is by using mixed solvents.61–66 In the mixed-solvent
method, the polymer nanowires are driven by using a combina-
tion of good and bad solvents. The polymers are usually dis-
solved in a good solvent. Then a small quantity of bad solvent is
added to the solution. The mixed solvent methods are based on
the idea that the unfavourable interactions between the polymer
chains and the bad solvent can induce the aggregation and self-
assembly of the polymer chains. Therefore, the amount of the
bad solvent in the solution will determine the degree of crystal-
lization of the polymers. Fig. 9 shows an example of making
P3HT nanowires using the mixed-solvent method. The P3HT
polymers are first dissolved in a good solvent. After the bad
solvent is added, P3HT polymers start to aggregate and P3HT
nanowires are formed.
This journal is ª The Royal Society of Chemistry 2011
Fig. 9 Schematic illustration of using the mixed-solvent method to make
P3HT nanowires. The P3HT polymers are first dissolved in a good
solvent. After the bad solvent is added, P3HT polymers start to aggregate
and P3HT nanowires are formed.
Publ
ishe
d on
24
Aug
ust 2
011.
Dow
nloa
ded
on 2
5/04
/201
4 02
:09:
05.
View Article Online
The mixed-solvent method was used by Kiriy et al. to fabricate
one-dimensional polyalkylthiophene aggregation in dilute solu-
tion by adding a poor solvent to the solution.61 Hexane, a good
solvent for alkyl side chains but a poor solvent for polythiophene
backbones, was used and the polyalkylthiophene formed ordered
main-chain collapse driven by the solvophobic interaction.
Length of the polyalkylthiophene aggregation can be adjusted by
the concentration of polyalkylthiophenes or the solvent
composition.61 The solar cell performance of these poly-
alkylthiophene aggregations was not reported. Moul�e et al. then
used a similar mixed-solvent method to determine the agglom-
erated–amorphous ratio of P3HT and to control the degree of
agglomeration/crystallinity of P3HT in the P3HT/PCBM solar
cell mixtures.62 The advantage of this method is that the filtering
is not required to obtain pure agglomerated P3HT, and further
heat-treatment of the polymer is not required. By adding nitro-
benzene as the dipolar solvent, the ratio of the P3HT in the
amorphous and aggregated phases was controlled and P3HT
aggregates were formed. A power conversation efficiency of 4%
(AM 1.5G) was achieved based on the P3HT/PCBM solar cell
mixtures with no pre- or post-treatment steps.62
The mixed solvent method was also used by Li et al. to make
ordered aggregates of P3HT, and the crystallinity of P3HT was
substantially increased.63 Hexane, a poor solvent for P3HT, was
titrated into well-dissolved P3HT-o-dichlorobenzene (ODCB)
solution. A power conversion efficiency of 3.9% was achieved
based on a P3HT:PCBM composite using this method, almost
four times than that of the pristine device.63 Zhao et al. also
prepared P3HT solution containing crystalline P3HT nanofibers
by adding a small amount of acetone into the solution.64 The
solvatochromic phenomenon in the P3HT was observed when
acetone was added, and the solution color shifted gradually from
orange to dark purple with more acetone content. The best
power conversion efficiency of 3.60% was achieved based on the
solar cell devices with the P3HT:PCBM composites prepared
from the chlorobenzene solution containing 2.5% acetone. This
value was higher than 3.45% from the control device, showing
that the hole transport is enhanced by adding small amount of
P3HT nanofibers because of the good connectivity of P3HT
nanofibers, and the demixing of PCBM is not influenced.64
Using the mixed solvent method, Kim et al. studied the soni-
cation-assisted self-assembly of P3HT nanowires with PC61BM
in a cosolvent system containing acetonitrile as the polar
solvent.67 The self-assembly of P3HT nanowires was found to
depend on the regioregularity of P3HT, solvent polarity, and
ultrasonic irradiation. A power conversion efficiency of 4.09%
This journal is ª The Royal Society of Chemistry 2011
was achieved based on the thermally annealed solar cells having
the self-assembled 98% regioregular P3HT nanowires/PC61BM
by the sonication-assisted self-assembly.67
In order to prepare well-controlled nanoscale morphologies in
the P3HT nanowires/PCBM film, Kim et al. described a two-step
process.65 The first process was the in situ formation of self-
organized P3HT nanowires by adding the marginal solvent,
cyclohexanone, to the blend solution in chlorobenzene. The
second process was the nanoscale phase separation achieved by
mild thermal annealing. This dual process effectively reduced the
interference between P3HT crystallization and phase separation.
The nanoscale PCBMdomains were developed in the second step
and bicontinuous percolation pathways between the P3HT and
PCBM components were produced. The power conversion effi-
ciency of 4.04% was achieved based on the P3HT nanowires/
PCBM film fabricated by the two-step process, with a photo-
current density of 10.9 mA cm�2 and a fill factor of 62.1%.65
Different from previous methods, Sun et al. reported an
unusual mixed-solvent approach to prepare P3HT nanofibers.66
P3HT was dissolved in a large quantity of marginal solvent with
a small amount of good solvent (chlorobenzene). The effects of
two marginal solvents, p-xylene and anisole, on the morphology
and solar cell performances were investigated. At room temper-
ature, anisole has a poorer solvent quality to P3HT compared
with p-xylene and was found to promote a higher degree
formation of P3HT nanofibers. A 50% improvement of the
power conversion efficiency was observed for the P3HT nano-
fibers by adding chlorobenzene into P3HT/anisole system than
the pure anisole system.66
2.2 Conjugated polymer nanoparticles
Conjugated polymer nanoparticles have also been applied to
fabricate organic solar cells. There are several approaches for
making polymer nanoparticles including reprecipitation, emul-
sion polymerization, microfluidic-assisted synthesis, and mini-
emulsion. The reprecipitation and miniemulsion methods are the
two most commonly used methods to make conjugated polymer
nanoparticles.68–70
The reprecipitation method was first developed by Kasai
et al.71 The particle formation is controlled by nucleation and
growth, or spinodal phase separation. The sizes of polymer
nanoparticles are controlled by the water temperature and the
concentration of the polymer solution. This method was used by
Kurokawa et al. to make nanoparticles of poly(thiophene).68 In
their studies, polymer solution was injected into vigorously stir-
red DI water by using a microsyringe. Szymanski et al. used
a similar reprecipitation method to make different conjugated
polymer nanoparticles such as MEH-PPV.69,70 Another modified
reprecipitation method was developed by Yabu et al. and regular
sized polymer nanoparticles were obtained.72 In the modified
method, a small amount of poor solvent (water) was slowly
dropped into a polymer solution (polystyrene in THF). After the
good solvent (THF) evaporated, the dissolved polymer precipi-
tated as fine particles.72
The second common approach to generate conjugated poly-
mer nanoparticles is by the miniemulsion process.73–77 In the
miniemulsion process, the polymers are first dissolved in a suit-
able solvent, followed by adding of water containing a small
Polym. Chem., 2011, 2, 2707–2722 | 2713
Publ
ishe
d on
24
Aug
ust 2
011.
Dow
nloa
ded
on 2
5/04
/201
4 02
:09:
05.
View Article Online
amount of a suitable surfactant. After sonication for a short
time, a homogeneous size distribution of the droplets of the
resulting miniemulsion is reached, and the whole mixture is
sonicated. Finally, a stable, aqueous dispersion of polymer
nanoparticles is obtained after the evaporation of the organic
solvent by gentle heating.
The miniemulsion technique was used by Kietzke et al. to
prepare polyfluorene nanoparticle blends for solar cell devices.76
The polymer used are poly(9,9-dioctylfluorene-2,7-diyl-co-bis-N,
N0-(4-butylphenyl)-bis-N,N0-phenyl-1,4-phenylenediamine) (PFB)
and poly-(9,9-dioctylfluorene-2,7-diyl-co-benzothiadiazole)
(F8BT). They applied two different approaches which both
involve using thin spin-coated layers. For the first approach,
heterophase solid layers were prepared from the mixing of two
dispersions of single-component nanospheres. For the second
approach, each individual particle contained both polymers.
Polymer solar cells based on the polymer nanoparticles made by
these two methods were fabricated, and device efficiencies were
found to be comparable to those of solar cells prepared from
solution. The device efficiencies were also found to be indepen-
dent of the choice of solvent which was used in the miniemulsion
process.76 Kietzke et al. also fabricated solar cells based on the
single- and dual-component polymer nanoparticles to further
study the dependence of the solar cell efficiency on the layer
composition.77 They found that an external quantum efficiency
of 4% was achieved for the solar cell device based on polymer
blend nanoparticles containing PFB:F8BT at a weight ratio of
1 : 2 in each individual nanoparticles. The authors proposed that
the small exciton diffusion length of F8BT and the tendency of
F8BT to penetrate the PFB phase are the most important factors
for the efficiency of these devices.77
Using similar methods for generating polymer nanoparticles,
Snaith and Friend reported an electroplating technique to form
a continuous uniform film of polyfluorene nanoparticles on
conductive and polymer-coated substrates.78 By spin-coating an
F8BT layer on top of the PFB:F8BT nanoparticle film, a multi-
layer structure was formed. A solar cell device based on the
multilayer structure was fabricated, and an open-circuit voltage
of 0.75 V was observed for the device based on the electroplated
nanoparticles.78 In addition to polyfluorene nanoparticles, Tada
and Onoda demonstrated the preparation of stable colloidal
suspensions of different materials such as C60, MEH-PPV, and
poly(3-alkylthiophene).79 A small amount of solution of target
materials was injected into a large amount of nonsolvents. These
organic particles can be collected by electrophoretic deposition
to make nanostructured films.79
Fig. 10 Strategies for using block copolymers for organic solar cell
applications.
2.3 Block copolymers
Block copolymers are polymers that consist of covalently-linked
blocks, and they have aroused great interest in recent years
because of their nanoscale self-assembly.80,81 Depending on the
number of blocks, the relative block volume fraction, and the
interaction parameters between the blocks, block copolymers can
self-assemble into a variety of ordered morphologies such as
sphere, cylinder, or lamella. External fields such as electric fields
or solvent annealing have been employed to orient the micro-
domains of block copolymers.82–84 Block copolymers are very
useful in organic solar cell applications.85 In this review, we will
2714 | Polym. Chem., 2011, 2, 2707–2722
focus on the use of diblock copolymers, which are comprised of
two chemically distinct blocks. For the applications in organic
solar cells, lamella and cylinder morphologies are usually the
preferred morphologies, because they provide the percolation
pathways for charge transport. As shown in Fig. 10, there are
several strategies for using the block copolymers in the applica-
tion of organic solar cells, which will be discussed below.
2.3.1 Donor–acceptor block copolymers. For the application
of organic solar cells, the simplest case is to use a donor–acceptor
block copolymer. The self-assembly of the block copolymers
create the interface needed for the electron dissociations. It was
demonstrated by Lindner et al., who studied a single-active-layer
solar cell device from block copolymers, poly(bisphenyl-4-
vinylphenylamine)-block-poly(perylene diimide acrylate)
(PvTPA-b-PPerAcr).86 The electronic functionalities were
attached as side groups instead of as main chains. The perfor-
mance of the solar cell devices based on the block copolymers
was increased by one order of magnitude with respect to effi-
ciency compared with the devices based on the blend of the two
individual segment polymers.86 This improved performance was
suggested to be caused by the larger phase-separated donor–
acceptor interface in the block copolymers.86,87
Later, Sommer et al. reported the synthesis and characteriza-
tion of two block copolymers, poly(bis(4-methoxyphenyl)-40-vinylphenylamine)-block-poly(perylene diimide acrylat)
(PvDMTPA-b-PPerAcr) and poly(N,N0-bis(4-methoxyphenyl)-
N-phenyl-N0-4-vinylphenyl-(1,10-biphenyl)-4,40-diamine)-block-
poly(perylene diimide acrylat) (PvDMTPD-b-PPerAcr).87 The
hole-conducting blocks bis(4-methoxyphenyl) phenylamine
(DMTPA) and N,N0-bis(4-methoxyphenyl)-N,N0diphenyl-(1,10biphenyl)-4,40diamine (DMTPD), are more electron-rich
than bisphenyl-4-vinylphenylamine (TPA). The new hole-con-
ducting blocks were used to vary the HOMO of the donor and to
improve the hole-carrier mobility. The power conversion effi-
ciency of the solar cell based on PvDMTPA-b-PPerAcr was
improved to 0.32%.87
P3HT has also been synthesized into the block copolymers for
use in organic solar cells. For example, Zhang et al. synthesized
a donor–acceptor diblock copolymer in which the electron donor
block is regioregular P3HT and the electron acceptor block is
poly(perylene diimide acrylate) (PPDA), a polyacrylate with
pendent perylene diimide groups.88 In the solid state, the copol-
ymers showed efficient photoluminescence quenching, indicating
the charge separation. The power conversion efficiency of the
solar cell based on the block copolymer was achieved at 0.49%
This journal is ª The Royal Society of Chemistry 2011
Publ
ishe
d on
24
Aug
ust 2
011.
Dow
nloa
ded
on 2
5/04
/201
4 02
:09:
05.
View Article Online
using AM 1.5G solar simulation.88 Lee et al. then synthesized
a new, well-defined diblock copolymer (P3HT-b-C60) based on
regioregular P3HT and fullerene.89 The P3HT-b-C60 showed
phase separation when the block copolymer was thermally
annealed. Complete photoluminescence quenching of the P3HT-
b-C60 film was observed, while the P3HT/PCBM blend still
showed some photoluminescence, indicating that the charge
transfer between P3HT and C60 are more effectively in the P3HT-
b-C60 diblock copolymer than that in the P3HT/PCBM blend.
Tao et al. reported the synthesis of rod–coil block copolymers
poly(3-hexylthiophene)-b-poly(n-butyl acrylatestat-acrylate per-
ylene) containing electron donor poly(3-hexylthiophene) and
acceptor perylene.90 The self-assembled morphology of the block
copolymers and the performances of the solar cells based on the
self-assembled block copolymers were studied. The authors
concluded that the device performance can be significantly
improved from well-defined interfaces but poorly organized
nanostructures.90
2.3.2 Block copolymer with one sacrificial block. The second
strategy for the application of block copolymers in organic solar
cells is to use a diblock copolymer with one conjugated block and
one sacrificial block. This strategy relies on the self-organization
of block copolymers to generate idealized morphologies. Upon
removal of the sacrificial block after the microphase separation,
nanopores or nanogaps are formed with the presence of the
conjugated block (donor).91 Then the second active material
(acceptor) can be filled into the nanopores or nanogaps to form
an ordered bulk heterojunction. The most common examples are
based on the use of block copolymers with one block of P3HT
and a sacrificial block.92 McCullough et al. discovered that end-
functionalized P3HTs can be synthesized with low polydispersity
using a catalyst-transfer polycondensation method.93,94 They
further showed that block copolymers of P3HT can be synthe-
sized by using the end-functionalized P3HTs via atom transfer
radical polymerization (ATRP).95 Using a similar method
combined with anionic polymerization, Dai et al. synthesized
copolymers consisting of P3HT and poly(2-vinylpyridine)
(P2VP) that microphase-separated and self-assembled into
different nanostructures of sphere, cylinder, and lamella.
Different nanofiber structures were also observed depending on
the volume fraction of the P2VP block.96
For the application of organic solar cells, Botiz et al. demon-
strated the synthesis of poly(3-hexylthiophene)-block-poly-
(L-lactide) (P3HT-b-PLLA) linear diblock copolymer as
a structure-directing agent for patterning active materials into
ordered nanostructures.97 The block copolymers self-assembled
to form a lamellar morphology, determined by the molecular
weights of the two blocks. The biodegradable PLLA block was
selectively removed by NaOH solution after the ordered micro-
phase-separated morphology was obtained. Acceptor material
fullerene hydroxide (C60) was then filled the gaps between the
P3HT domains by dip-coating technique.97 One challenge using
this strategy is the lateral collapse of the resulting P3HT nano-
structures after removing the PLLA. Botiz et al. further
addressed this issue by proposing that collapse and the subse-
quent aggregation of the P3HT domain is because of the capil-
lary forces present between the nanodomains.98 They applied
different drying approaches to control the surface tension forces
This journal is ª The Royal Society of Chemistry 2011
generated during the liquid evaporation process. A reduction in
domain collapse was observed and the molecular ordering in the
plane perpendicular to the substrate was improved.98
2.3.3 Block copolymer with two sacrificial blocks. Block
copolymers can also be used simply as a template, where both
blocks in a diblock copolymer are sacrificial blocks. One of the
blocks will be selectively removed to create spaces that could be
filled with the conjugated polymers as the donor. The second
sacrificial blocks will then be removed, followed by the deposi-
tion of the acceptor materials. Therefore, the block copolymers
are not present in the final devices. The advantage of this strategy
is that well-ordered block copolymers such as the common poly
(styrene)-block-poly(methyl methacrylate) (PS-b-PMMA) can be
used and general methods to align the block copolymer domains
can be applied. In addition, the problem that the crystalline
behaviour of most conjugated polymers can affect the micro-
phase-separation of block copolymers is avoided by using
amorphous block copolymers as templates.
Conducting polymer nanorods were prepared by Lee et al.,
who used a porous diblock copolymer as a template.99 In their
work, a mixture of poly(styrene)-block-poly(methyl methacry-
late) (PS-b-PMMA) and a poly(methyl methacrylate) (PMMA)
homopolymer is spin-coated onto a random copolymer-coated
ITO substrate. Nanopores were generated after the PMMA
homopolymer was removed using a selective solvent. The arrays
of conducting polypyrrole (PPy) nanorods were fabricated
directly on the indium-tin oxide coated glass by an electro-
polymerization.99 The PPy nanorods were shown to have much
higher conductivity than that of thin PPy films, which is caused
by the high degree of chain orientation. The block copolymer
template was removed by using a suitable solvent, resulting in
self-supporting arrays of conducting polymers oriented normal
to the substrate surface. This method will be useful considering
the well-controlled conjugated polymer nanorods. The block
copolymer templates can be filled with other conjugated poly-
mers such as P3HT, and solar cells devices can be made after the
removing of the second sacrificial block, followed by the evap-
oration of other active materials such as PCBM.
2.3.4 Block copolymers as compatibilizers. Instead of being
used as the active materials, block copolymers can be used as
compatibilizers in organic solar cells. In the P3HT/PCBM
system, a thermal annealing step above the glass transition
temperature (Tg) of the active material is usually applied to
improve the device performance.27,28 But phase segregation also
occurs during the annealing process, affecting the resultant effi-
ciency. The block copolymers can be used as compatibilizers to
lower the interfacial energy between donor acceptor domains and
can affect the phase segregation behaviour of the active
materials.
Amphiphilic diblock copolymers incorporating fullerene and
P3HT macromonomer was synthesized by Sivula et al. for their
uses as compatibilizers in P3HT/PCBM based solar cells.26 Phase
segregation of P3HT and PCBM domains were found to be
altered and even prevented by using the compatibilizers. Yang
et al. also synthesized rod–coil block copolymers based on P3HT
containing C60 chromophores (P3HT-b-P(SxAy)-C60) and used
the block copolymer as a surfactant for the P3HT/PCBM based
Polym. Chem., 2011, 2, 2707–2722 | 2715
Publ
ishe
d on
24
Aug
ust 2
011.
Dow
nloa
ded
on 2
5/04
/201
4 02
:09:
05.
View Article Online
solar cells.100 The interfacial morphology between P3HT and
PCBM domains was altered by adding a small amount of the
block copolymer (P3HT-b-P(SxAy)-C60). The efficiency of the
solar cell was improved to 3.5%, which was 35% increase
compared with the standard device without adding the block
copolymer.100
Fig. 11 Schematic illustration of using the nanoimprint lithography to
make polymer nanostructures. A polymer film is first coated on
a substrate. A master mold is used to imprint the polymer film at elevated
temperatures to transfer the patterns. Polymer nanostructures are
released after the demolding process.
2.4 Layer-by-layer deposition
Organic solar cells based on multi-layered small molecule films
have been fabricated using vacuum deposition. For example, the
power conversion efficiency of 3.6% was achieved from a simple
donor–acceptor bilayer cell based on copper phthalocyanine
(CuPc) and fullerene (C60).101 Even though the vacuum deposi-
tion technique is useful for precisely controlling the film thick-
ness, this technique usually requires high-vacuum and high
temperatures conditions, which is not suitable for polymer
materials.102 Therefore, multi-layered solar cells based on poly-
mers are relatively less studied. In addition, it is usually difficult
to find suitable selective solvents for making multi-layered
polymer films to avoid the dissolving problem.
Layer-by-layer (LbL) approaches have been one of the best
methods to generate conductive multilayer films based on
conjugated polymers.103–106 These multilayer films are fabricated
in organic solvents and are useful in different optoelectronic
applications such as the organic solar cells. Liang et al. reported
the synthesis and fabrication of hybrid multilayer thin films of
poly(p-phenylenevinylene)s (PPVs), and CdSe nanoparticles by
using the layer-by-layer assembly approach.106 This method
enables the preparation of hybrid thin films with great stability
through covalent coupling reactions between polymers and
nanoparticles. The power conversion efficiency of 0.71% (10 mW
cm�2) was achieved from the solar cells based on the PPV/CdSe
nanocomposites.106 Solar cells based on the self-assembled layers
of functional polymers and inorganic particles using the layer-
by-layer method was also demonstrated by Kniprath et al.107
Alternating layers of the polythiophene derivative, poly(2,3-
thienyl-ethoxy-4-butylsulfonate) (PTEBS), and TiO2 nano-
particles were prepared using the layer-by-layer methods. Solar
cells based on the composites films were fabricated and stable
photovoltaic behavior was observed with photovoltages of up to
0.9 V.107 Hybrid films of polymers and lead selenide nanocrystals
(PbSe-NCs) using the layer-by-layer deposition technique was
reported by Vercelli et al.108 PbSe-NCs and sulfonate-, carbox-
ylate-, and pyridine-based polymers were alternately deposited
on ITO-glass surfaces. The hybrid film was expected to be useful
for organic-inorganic solar cells in the infrared region of the solar
spectrum.108
Benten et al. reported the preparation of poly(p-phenyl-
enevinylene) (PPV) by using the layer-by-layer deposition of the
PPV precursor cation and poly(sodium 4-styrenesulfonate) (PSS)
and studied their applications in solar cells.102 Multilayer poly-
mer solar cells were fabricated based on the layer-by-layer
assembled PPV layer. In their work, the PEDOT:PSS spin-
coated film was cross-linked by gelation, yielding insoluble film
in the polyelectrolyte aqueous solutions. The best power
conversion efficiency of the solar cells based on PEDOT:PSS/
PPV/C60 was achieved at 0.26% (AM 1.5G, 100mW cm�2, air),
2716 | Polym. Chem., 2011, 2, 2707–2722
where the thickness of the PPV layer was 11 nm, comparable to
the diffusion length of the PPV singlet exciton.102
2.5 Nanoimprint lithography
As shown in Fig. 11, nanoimprint lithography (NIL) uses a mold
with nanostructures that can be transferred into a polymer by
imprinting the mold onto the polymer.109 The fabrication of the
nanostructures on the mold is mostly done by e-beam lithog-
raphy and is time-consuming and expensive. But these molds can
be repeatedly used, so the mold can be applied in an inexpensive
and fast process. The NIL process can be used for small areas,
but roll-to-roll processing and imprinting for large area are also
feasible. NIL has been employed by several groups to produce
nanostructures in the application of organic solar cells.110–112 For
polymer-based solar cell devices, the device performances are
improved when nanoscale structures are imprinted into the
donor polymer film, followed by the evaporating or spin-coating
of acceptor materials.
Nanopatterns of polythiophene derivatives on flexible plastic
were demonstrated by Kim et al. using the nanoimprint lithog-
raphy.110 Solar cell devices were fabricated after spin-coating
PCBM and evaporating Al. The short-circuit current of the
nanoimprinted devices was shown to increase with the interfacial
area of the structures. The fill factor and power conversion effi-
ciency also increased with the interfacial area.110
High density periodic P3HT nanopillars (�1010 cm�2) were
reported by Aryal et al. using nanoimprint lithography.111 A
large-scale nanoporous Si mold was obtained by using an anodic
alumina template as a mask for a two-step plasma etching
process. The pores in the Si mold were 50–80 nm wide and 100–
900 nm deep and the pore depth and profile can be controlled by
adjusting the plasma etching conditions such as Cl2 : Ar ratio,
time, pressure, and bias power. Ordered and high density poly-
mer nanopillars including poly(methyl methacrylate) (PMMA)
and P3HT were fabricated by this method.111 The imprinted
P3HT nanopillars were then used for organic solar cell devices
followed by depositing PCBM on top of the nanostructures. For
the purpose of comparison, double layer nonpatterned solar cell
devices without the imprinting steps using similar process were
fabricated. The power conversion efficiency (AM 1.5G, 100 mW
cm�2) was improved by 78% from the solar cell devices built on
imprinted P3HT pillars compared with the devices of the non-
patterned layer. The fill factor and short-circuit current also
showed 38% and 20% increases, respectively. The authors
concluded that the main reason for the improved performance of
the solar cell devices was due to the well-controlled interdigitized
This journal is ª The Royal Society of Chemistry 2011
Publ
ishe
d on
24
Aug
ust 2
011.
Dow
nloa
ded
on 2
5/04
/201
4 02
:09:
05.
View Article Online
heterojunction morphology which allowed more efficient charge
transport and exciton dissociation.111
For the imprinted P3HT nanostructures, Aryal et al., also
studied the crystallization and chain orientation by using the out-
of-plane and in-plane grazing incident X-ray diffraction.113 The
geometry (gratings or pillars) of these nanostructures was shown
to have a strong influence on the 3D chain alignment of the
nanostructures. The chain orientations inside nanoimprinted
P3HT gratings and nanopillars were proved to be vertical by in-
plane and out-of-plane grazing incidence X-ray diffraction
(GIXRD) measurement.113 The chain alignment was proposed to
be induced by the nanoconfinement effect during the imprinting
process through p–p interaction and the hydrophobic interac-
tion between polymer chains and the surfaces of the molding
materials. The vertical chain alignment of P3HT chains in both
nanogratings and nanopillars also indicate their potentials for
improving charge transport and solar cell devices compared with
bulk heterojunction solar cells.113
In addition to P3HT nanopillars, nanoimprinted P3HT nano-
gratings were demonstrated by Zhou et al.114 The nanoimprinted
P3HT nanogratings were fabricated by imprinting a P3HT film
using a Si mold gratings of 100 nm in width, 100 nm in depth, and
200 nm in pitch. PCBM was then spin-coated on top of the
nanograting as the electron transport material. An orthogonal
solvent (dichloromethane) that dissolves PCBM well but not the
P3HTwasused toallow the stackingofPCBMon topof theP3HT
nanostructures, resulting in nondistortion of the nanostructures.
For comparison studies, two types of nonpatterned films of P3HT
andPCBMwere also fabricated as solar cell devices. The first type
of nonpatterned film was made by spinning a 50 nm PCBM layer
on top of an 85 nm thick P3HT layer. Lee et al. have shown that
this kind of film is not a bilayer, but an intermixed hetero-
junciton.37 The second type of nonpatterned film was made by
spin-coating a mixture of P3HT/PCBM (1 : 0.9) onto the
substrate. The power conversion efficiency (AM 1.5G, 100 mW
cm�2) of the solar cell with P3HT nanogratings was higher than
those with the nonpatterned films.114 The fill factor and short-
circuit current also increased compared with those of the bilayer
and blended solar cells. The authors concluded that the improved
performance in P3HT nanograting devices was due to the better
chain alignment in P3HT nanogratings and the increase in the
interfacial area. The favorable 3D chain alignment enables high
mobility, resulting in improved current density, fill factor, and
efficiency of the nanoimprinted solar cells. The authors also
proposed that better device performance can be expected by
increasing the density and the aspect ratio of nanogratings.114
To further improve the performance of P3HT nanogratings
based solar cells, Yang et al. used oblique deposition to deposit
C60 into the P3HT nanogratings to have good infiltration of C60
into the nanoimprinted P3HT nanogratings.115 The evaporation
angle of C60 was found to affect the uniformity and step coverage
of C60 infiltration into the P3HT nanostructures. At the optimal
condition for the C60 deposition filling, increased exciton disso-
ciation rate at the larger P3HT-C60 interfacial area was achieved
and the power conversion efficiency was observed to increase
50%, compared with flat bilayer devices. The authors also
expected that there will be 200% enhancement in power
conversion efficiency of the solar cell devices if the P3HT gratings
can be twice as their width.115
This journal is ª The Royal Society of Chemistry 2011
Nanoimprinting was combined with lamination technique by
Wiedemann et al. to make nanostructured bilayer devices.116 In
their work, P3HT was nanoimprinted using an anodic aluminum
oxide as a stamp. PCBM was then deposited via a lift-off lami-
nation technique.116 Instead of forming the P3HT nanostructures
first followed by depositing PCBM, Na et al. used a soft litho-
graphic approach to prepare periodic patterned P3HT/PCBM
structures.117 Photo-induced surface-relief gratings on azo poly-
mer films and poly(dimethylsiloxane) were used as a master and
stamp. The power conversion efficiency of the patterned devices
was 4.02%, compared with 3.56% from the unpatterned
devices.117 To avoid the nanoimprinting process at high
temperatures, Shih et al. used textured Si wafer as a stamp to
nanoimprint P3HT:PCBM blends at room temperature, and the
Voc, Jsc, and FF of the nanoimprinted devices was found to
increase by 50%.118
To achieve the interpenetrating nanostructures, He et al.
demonstrated a double nanoimprinting method to create nano-
patterned polymer blends with features in the 25 to 200 nm size
range.119 The solar cell devices based on the nanostructured
polymer blends of poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis
(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-20,20 0-diyl) (F8TBT)/
poly(3-hexylthiophene) (P3HT) were fabricated and the influence
of feature size on device performance was investigated. The solar
cell devices were shown to have extremely high densities (1014
mm�2) of interpenetrating nanoscale columnar features in the
active polymer layer.119 The power conversion efficiency of the
solar cell was improved to 1.9%, primarily due to the improved
fill factor over that from the solution demixed device. The poorly
controlled morphology formed from the demixed polymer blend
layer was considered to cause the avoidance of locally trapped
photogenerated charges, resulting in lower efficiency.119 The
same technique was also applied to polymer-fullerene based solar
cell devices.112 The AFM and SEM results showed that the
features in the polymer mold used in the second imprinting
process were only slightly distorted during imprinting steps. The
performances of the solar cell devices using P3HT or F8TBT as
the donor materials and PCBM as the acceptor material were
investigated. In both cases, the performances of the devices were
influenced by the feature size and the interfacial area.112 Different
from other imprinting methods, which have the disadvantages of
rather large imprinted feature sizes and high temperature depo-
sition process, the advantage of the new double nanoimprinting
method is that the pure form of any materials can be used. The
requirement for the donor and acceptor materials is that they
have a small difference in glass transition temperature (Tg),
melting temperature (Tm), or solubility in selective solvents.
Otherwise, the first imprint might be erased during the second
imprinting step. An additional advantage of this double nano-
imprint method is that both donor and acceptor layers can
completely cover the electrodes.112,119
The most popular choices for the donor and acceptor in the
nanoimprinting method are still P3HT and PCBM. Instead of
using PCBM as the electron acceptor, N,N0-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C13) was used by
Cheyns et al. They first obtained trenches of poly(3-hexylth-
iophene) (P3HT) with feature size down to 50 nm and aspect
ratio up to 2 by nanoimprinting.109 Then the electron acceptor
(PTCDI-C13) was deposited from the vapor phase. Zinc oxide
Polym. Chem., 2011, 2, 2707–2722 | 2717
Publ
ishe
d on
24
Aug
ust 2
011.
Dow
nloa
ded
on 2
5/04
/201
4 02
:09:
05.
View Article Online
(ZnO) nanoparticles was later spin-coated on top to reduce the
roughness of the layered structures. A 2.5-fold enhancement of
the short-circuit current was observed in the nanoimprinted
devices compared with the planar devices.109 Another electron
acceptor 4,7-bis(2-(1-ethylhexyl-4,5-dicyano-imidazol-2-yl)
vinyl)benzo[c]1,2,5-thiadiazole (EV-BT) was used by Zeng
et al.120 They applied nanoimprinting for bilayer solar cells using
P3HT as the electron donor and EV-BT as the electron acceptor.
The power conversion efficiency of the devices was increased to
0.3% for the nanoimprinting devices, compared with 0.2% for the
simple bilayer device.120
2.6 Template methods
Nanoporous templates have been widely used to make one-
dimensional polymer nanostructures.121–127 Template-based
methods represent straightforward routes to make nano-
materials, in which the template is simply used as a scaffold, and
the materials are fabricated and shaped by the geometry of the
template. Since the templates consist of cylindrical pores of
uniform diameter, monodisperse nanocylinders of the desired
material, whose dimensions can be carefully controlled, are
obtained in high yield within the pores of the template mate-
rial.125 Another advantage of using the template method is that
the placement and dimensions of different components of the
nanomaterials can be controlled. Depending on the material, the
chemistry of the pores wall, and other operating parameters,
these nanocylinders may be solid (a nanorod) or hollow (a
nanotube).123 Fig. 12 describes the basic concept of using the
template method. Different materials, such as polymers, can be
introduced into the nanopores of the templates by wetting the
pore walls of the porous template.128 The surface energy of the
wall is usually high and the precursor materials cover the surface
of the walls to reduce the surface energy by a simple wetting
process. The nanomaterials can remain inside the pores of the
templates or they can be released and collected by removing the
template selectively.
Various materials have been used as the porous templates, but
ion-track-etched membranes and anodic aluminum oxide (AAO)
Fig. 12 Schematic illustration of using the template method to make
polymer nanostructures. A polymer film is first deposited on a substrate.
Then a porous template is brought into contact with the template.
Polymers are able to wet the surface of the nanopores of the template by
a capillary force via thermal annealing. After removing the template by
a selective etching solution, polymer nanostructures are generated.
2718 | Polym. Chem., 2011, 2, 2707–2722
templates are the most commonly employed.129 Ion-tracked
membranes are commercially available as filters in a wide variety
of pore sizes.130 They are usually prepared from either polyester
or polycarbonate thin films with pore diameters ranging from 10
to 2000 nm. However, the track-etched membranes have low
porosities and contain randomly distributed nanochannels
across the membrane surface. In addition, the random nature of
pore formation during this process can result in a large number
of intersecting pores that will harmfully affect the homogeneity,
number, and sizes of the synthesized nanomaterials.125
Another important template is anodic aluminum oxide (AAO)
template which is prepared electrochemically from aluminum.
The popularity of the AAO template is because of its exceptional
thermal stabilities, ease of fabrication, and its regular pore
diameter and pore length.131 The AAO templates typically
contain high porosities, and the pores are arranged in a hexag-
onal array. Pore densities as high as 1011 pores per square cen-
timetre can be achieved, and typical membrane thickness can
range from 10 to 100 mm.132 AAO templates of many diameters
(5–267nm) can be synthesized following the two-step anodization
process established by Masuda and co-workers in which nano-
sized pores are grown in an insulating oxide film of alumina.133,134
The template method usually requires removal of the template
with selective etching solution such as weak acid or weak base.
But a major challenge associated with these nanostructures is
associated with their lateral collapse, which is related to the high
aspect ratio of the nanostructures or the capillary force caused by
the evaporation of solvent.135,136 A freeze drying technique has
been applied to remove the aqueous medium after the etching
process to prevent the lateral collapse of the nanostructures.137
Polythiophene nanotube films were prepared by Cao et al.
using the anodic aluminium oxide template.138 A gold layer was
first deposited onto one side of the alumina membrane template
with a pore size of 20 nm. Then an ITO layer was contacted
closely to the gold layer as a current collector. By direct oxidation
of thiophene in boron trifluoride diethyl etherate (BFEE) solu-
tion, polythiophene nanotubes were synthesized in the nano-
pores of the AAO template.138 After dissolving the template in
1M KOH solution, the aligned nanotube film was obtained. In
their work, surface photovoltage spectrum (SPS) and field-
induced surface photovoltage spectrum (FISPS) were used to
investigate the electronic structure and the charge behaviour of
the polythiophene nanotube films. They observed two extra
photovoltaic responses in the near-IR region, besides the
intrinsic p–p* transition of polythiophene chains. From the
principle of FISPS and the band theory, the new responses were
attributed to the charged surface electronic states, which were
caused by the interaction between the nanotubes and the oxygen
absorbed on the surface.138 Electrochemical polymerization was
used by Huesmann et al., to synthesize polythiophene with AAO
templates.139 They showed that geometry of the prepared nano-
structures can be controlled by the hydrophobic side-chains, such
as 3-hexyl and 3-(2-ethyl)hexyl.
Wang et al. reported the use of porous alumina templates to
fabricate ordered arrays of core-shell P3HT and PCBM for the
use in organic solar cells.140 The P3HT/PCBM film with a thick-
ness of 120 nm was first spin-coated onto an ITO substrate. An
AAO template was then placed on top of the P3HT/PCBM film.
After annealing in an oven under vacuum for 6 h, core-shell
This journal is ª The Royal Society of Chemistry 2011
Publ
ishe
d on
24
Aug
ust 2
011.
Dow
nloa
ded
on 2
5/04
/201
4 02
:09:
05.
View Article Online
P3HT/PCBM composite nanorods were formed. The template
was dissolved by using 10 wt% NaOH solution and the nanorod
arrays were released. For the solar cell devices using the P3HT/
PCBM nanorods at a 1 : 0.6 ratio (w/w), the values of PCE and
Jsc were 2.0% and 8.7 mA cm�2, respectively.140 The performance
of the devices annealed at 120 �C was higher than that at 100 �Cbecause the phase separation of the nanorods at higher temper-
atures was more complete, resulting in better device perfor-
mance. The annealing effect induced the increase in the transport
properties, the degree of crystallization, and the light absorption
of the P3HT-rich region of the solar cells devices.140
In addition to the normal solar cell devices, Wang et al. also
fabricated inverted heterojunction solar cell devices incorpo-
rating the core/shell P3HT/PCBM nanorod arrays.141 The effi-
ciencies of these core/shell nanorod inverted solar cells were
higher than those of the corresponding conventional inverted
bulk heterojunction solar cell devices. The hole mobility of the
optimized nanorod arrays was over one order greater than that
of the conventional bulk heterojunction structure. The core-shell
nanorods provided a more efficient charge transport for the
devices, resulting in a higher short-circuit current density and
power conversion efficiency.141
Using the AAO template, Kim et al. demonstrated the fabri-
cation of P3HT nanorods which were aligned perpendicular to
the ITO substrate.142 The AAO membranes with a diameter of
50 nm, an aspect ratio of 3, and an interpore size of 100 nm were
synthesized by anodization of an aluminium sheet at 40 V in
0.3 M oxalic acid and were used as the templates. For the infil-
tration of the P3HT chains into the nanopores of the AAO
templates, an AAO template was placed on top of a P3HT film
and was heated to 250 �C for 30 min under vacuum. This
temperature was used because it is well above the melting
temperature of P3HT. The P3HT chains were drawn into the
nanopores of the template by capillary action. The template was
then removed by copper chloride (CuCl2) and sodium hydroxide
(NaOH) solution.142 They found that the chain orientations of
P3HT inside the nanorods were aligned in the direction normal
to the pore wall of the AAO templates. The electrical conduc-
tivity of the P3HT nanorods was enhanced up to ten times
compared with the conductivity of the continuous P3HT film,
mainly due to the preferred crystalline orientation. The power
conversion efficiency of the solar cell devices using P3HT
nanorods also increased from 0.17% to 1.12% compared with
devices using planar P3HT film. This 6.6 fold increase of effi-
ciency was attributed to the conductivity enhancement afforded
by the nanorods, more efficient charge separation, and a greater
interface area between the donor and acceptor materials.142
Instead of the AAO templates, Jeon et al. used polycarbonate
and PETmembrane films as templates to make nanostructures of
poly(3-octylthiophene) (P3OT) and fullerene.143 P3OT was
synthesized in the micropores of the membranes. The power
conversion efficiency of the heterojunction solar cells devices was
increased with increasing pore content.143
2.7 Nanoelectrodes
Conjugated polymer nanostructures can be made based on
patterned nanoelectrodes. Most works related to this topic are
based on the formation of nanorods of the anode, usually indium
This journal is ª The Royal Society of Chemistry 2011
tin oxide (ITO). The active polymer layers are then deposited on
top of the patterned nanoelectrodes, and conjugated polymer
nanostructures can be generated whose geometry is based on the
underlying nanoelectrodes. To obtain better device performance
using this strategy, one usually has to deal with the problem of
the incomplete filling of the polymers into the space created by
the nanoelectrodes.144
An oblique electron-beam evaporation method was demon-
strated by Yu et al. to fabricate indium-tin-oxide (ITO) nano-
rods.145 The randomly oriented ITO nanorods were deposited on
an ITO-coated glass substrate. After spin-coating of the hole-
transport layer of PEDOT:PSS, a mixture of regioregular P3HT
and PCBM with a weight ratio of 1 : 1 was used as the active
layer. In this work, a small nitrogen flow was introduced in order
to facilitate the segregation of indium during the nucleation
process, and the resulting column growth of the ITO nanorods in
an oxygen-deficient environment was therefore promoted.146 The
performance of the solar cells with ITO nanorods was compared
with that of the conventional solar cells. The results showed that
the solar cells with ITO nanorods represented an enhancement
factor of up to 10% compared with a conventional solar cell
under the AM 1.5G conditions.145 Instead of spin-coating, Hsu
et al. also used electrochemical deposition to deposit PEDOT:
PSS onto the ITO nanoelectrodes.146 The P3HT and PCBM
mixture was then spin-coated onto the PEDOT layer for the
fabrication of the solar cell devices. The performance of the solar
cells with nanoelectrodes was compared with solar cells with
planar electrodes, and a PCE of 3.02% was achieved for the solar
cell devices with the ITO nanoelectrodes.146
ITO nanoelectrodes can also be fabricated by using a glancing
angle deposition technique.147 In this deposition process, the
substrate is oriented at an oblique angle with respect to the
incident vapour flux. After the fabrication of ITO nano-
electrodes, PEDOT:PSS and P3HT/PCBM (1 : 1 in weight ratio)
were spin-coated on the substrate followed by the evaporation of
aluminium. The power conversion efficiency of solar cells with
the ITO nanorods was 2.2% which was a third greater than that
of solar cells with a commercial ITO.147 In addition, a silica
capping technique was used to prevent electrical shorts and to
reduce series resistance in the ITO nanorods-based solar cells. By
using this capping technique, a further improved power
conversion efficiency of 2.5% was observed.147
Using similar concepts, Fung et al. demonstrated the deposi-
tion of ITO nanorods on glass and glass/ITO substrates using
electron-beam deposition and sputtering methods.148 The depo-
sition times for both methods were very short and the substrate
was not necessary to be tilted. After spin-coating PEDOT:PSS
and P3HT/PCBM (1 : 1 in weight ratio) followed by the evapo-
ration of aluminium, solar cells using the ITO nanorods were
prepared. The power conversion efficiency of solar cells with the
bare ITO film is 2.1%. This value was significantly increased to
3.2% by using the solar cells with the sputtered ITO nanorods on
an ITO film.148
2.8 Porous inorganic materials
Porous inorganic materials such as titania (TiO2) provide
a convenient way to make conjugated polymer nanomaterials.
The conjugated polymers are synthesized or infiltrated into the
Polym. Chem., 2011, 2, 2707–2722 | 2719
Publ
ishe
d on
24
Aug
ust 2
011.
Dow
nloa
ded
on 2
5/04
/201
4 02
:09:
05.
View Article Online
nanopores of the porous inorganic materials. The advantage of
using this method is that the fabricated polymer nanomaterials
do not need to be released from the inorganic materials because
titania or some other inorganic materials already serve the
function as the electron acceptor. This simplified coating process
can avoid the problem that the conjugated polymers may
collapse or distort during the later coating or imprinting process
of the acceptor materials.112
Coakley et al. reported the preparation of films of titania with
a uniform distribution of pores sizes by dip coating the substrates
with a solution of a titania sol–gel precursor followed by calci-
nation at temperatures in the range of 400–450 �C.149 A poly
(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) tri-
block copolymer was used as the structure-directing agent. The
nanopores in the porous titania was filled with regioregular
P3HT by spin-casting a film of P3HT followed by heating at
temperatures in the range of 100–200 �C. The amount of polymer
that can be filled into the titania films depends on the tempera-
ture that is used for infiltration. Electron transfer from the
polymer to the titania was suggested from the PL measurement.
Later, Coakley et al. fabricated solar cells based on the meso-
porous titania/infiltrated-P3HT.150 The pore size of the meso-
porous titania films is slightly less than 10 nm, which is close to
the exciton diffusion length for many conjugated polymers. The
power conversion efficiency of 1.5% was achieved under mono-
chromatic 514 nm light based on the mesoporous titania/P3HT
composites.150
Porous titania with typically 33% volume can be produced by
using the poly(ethylene oxide)-poly(propylene oxide)-poly
(ethylene oxide) triblock copolymer as the structure-directing
agent.149 This ratio resulted in the maximum external quantum
efficiency of 10%. To increase the ratio of the nanopores in the
titania film, Wang et al. prepared titania interconnected network
structure with large pores using polystyrene-block-polyethylene
oxide (PS-b-PEO) diblock copolymer as the templating
agent.151,152 The pore size can be controlled by the amount of Ti
precursor. Poly(2-methoxy-5-(20-ethyl-hexyloxy)-phenylenevinylene) (MEH-PPV) was infiltrated into the nanoporous
titania and solar cell devices were fabricated. The device
performance was improved with a short-circuit current of
3.25 mA cm�2 under AM 1.5 solar illumination.151
Well-ordered nanoporous titania can be fabricated by using
2D surface relief gratings (SRGs) on polymer films as
a template, as reported by Kim et al.153 In their work, solution
of MEH-PPV in chlorobenzene was spin-coated onto the
nanoporous titania followed by a baking process. The efficiency
of the solar cells was increased from 0.05% for the flat titania
film to 0.21% for the well-ordered nanoporous titania film.
Another way to make highly ordered nanoporous titaina was
demonstrated by Her et al., who applied the nanoimprinting
method.154 The well-ordered nanoporous titania was fabricated
by embossing the sol–gel mixture of titanium ethoxide with an
array of PMMA nanopoles, followed by a sintering process at
500 �C. P3HT was used as the electron donor and was spin-
coated onto the nanoporous titania. The power conversion
efficiency was improved to 0.16% (AM 1.5G, 100 mW cm�2)
for the solar cells based on nanostructured titania/P3HT
composites, compared with 0.09% for the solar cells based on
flat titania films.154
2720 | Polym. Chem., 2011, 2, 2707–2722
3 Conclusions and outlook
Polymer-based solar cells have attracted considerable interest in
recent years because of their low cost and easy fabrication
process. To achieve the goal of commercialization, researchers
have been working on many approaches to improve the device
performance. Nanostructures provide large donor–acceptor
interfaces for effective exciton dissociation and continuous paths
for efficient charge transport. In this review, we provide an
overview of different techniques for fabricating conjugated
polymer nanostructures for the application in organic solar cells.
Novel techniques or combination of fabrication techniques are
expected to be developed to optimize the device performance.
Although significant processes have been made in this field by
using conjugated polymer nanostructures, there are still many
challenges to overcome. The molecular orientations and
conformations in the nanostructures are different from those in
the bulk, especially when the size of the geometry is close to
several nanometres. It is necessary to understand the dependence
of the optoelectronic properties of the conjugated polymers on
the size of the geometry in the molecule level. In addition, the
arrangement of the donor and the acceptor at the interface
requires precise control by manipulating factors such as the
interfacial interaction. The optimized device performance will
rely on the interdisciplinary work involving material synthesis,
morphology control, structure design, and device fabrication.
With these efforts, 10% or higher efficiencies are achievable for
polymer solar cells to be a renewable energy source.
Acknowledgements
We thank the National Science Council of the Republic of China
for financial support.
References and notes
1 M. Gratzel, Chem. Lett., 2005, 34, 8–13.2 P. V. Kamat, J. Phys. Chem. C, 2007, 111, 2834–2860.3 A. Shah, P. Torres, R. Tscharner, N. Wyrsch and H. Keppner,Science, 1999, 285, 692–698.
4 A. Goetzberger, C. Hebling and H. W. Schock, Mater. Sci. Eng., R,2003, 40, 1–46.
5 S. Gunes, H. Neugebauer and N. S. Sariciftci,Chem. Rev., 2007, 107,1324–1338.
6 B. C. Thompson and J. M. J. Frechet, Angew. Chem., Int. Ed., 2008,47, 58–77.
7 M. Yan, L. J. Rothberg, E. W. Kwock and T. M. Miller, Phys. Rev.Lett., 1995, 75, 1992–1995.
8 Y. J. Cheng, S. H. Yang and C. S. Hsu,Chem. Rev., 2009, 109, 5868–5923.
9 E. Bundgaard and F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2007,91, 954–985.
10 F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2009, 93, 394–412.11 K. M. Coakley and M. D. McGehee, Chem. Mater., 2004, 16, 4533–
4542.12 R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes,
R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. Dos Santos,J. L. Bredas, M. Logdlund and W. R. Salaneck, Nature, 1999, 397,121–128.
13 A. Kraft, A. C. Grimsdale and A. B. Holmes, Angew. Chem., Int.Ed., 1998, 37, 402–428.
14 T. D. Nielsen, C. Cruickshank, S. Foged, J. Thorsen andF. C. Krebs, Sol. Energy Mater. Sol. Cells, 2010, 94, 1553–1571.
15 J. Rostalski and D.Meissner, Sol. EnergyMater. Sol. Cells, 2000, 61,87–95.
This journal is ª The Royal Society of Chemistry 2011
Publ
ishe
d on
24
Aug
ust 2
011.
Dow
nloa
ded
on 2
5/04
/201
4 02
:09:
05.
View Article Online
16 A. Gadisa, M. Svensson, M. R. Andersson and O. Inganas, Appl.Phys. Lett., 2004, 84, 1609–1611.
17 G. Dennler, M. C. Scharber and C. J. Brabec, Adv. Mater., 2009, 21,1323–1338.
18 H. Sirringhaus, N. Tessler and R. H. Friend, Science, 1998, 280,1741–1744.
19 V. Shrotriya, J. Ouyang, R. J. Tseng, G. Li and Y. Yang, Chem.Phys. Lett., 2005, 411, 138–143.
20 F. L. Zhang, E. Perzon, X. J. Wang, W. Mammo, M. R. Anderssonand O. Inganas, Adv. Funct. Mater., 2005, 15, 745–750.
21 D. Muhlbacher, M. Scharber, M. Morana, Z. G. Zhu, D. Waller,R. Gaudiana and C. Brabec, Adv. Mater., 2006, 18, 2884–2889.
22 M. Jorgensen, K. Norrman and F. C. Krebs, Sol. EnergyMater. Sol.Cells, 2008, 92, 686–714.
23 K. W. Wong, H. L. Yip, Y. Luo, K. Y. Wong, W. M. Lau,K. H. Low, H. F. Chow, Z. Q. Gao, W. L. Yeung andC. C. Chang, Appl. Phys. Lett., 2002, 80, 2788–2790.
24 L. M. Chen, Z. R. Hong, G. Li and Y. Yang, Adv. Mater., 2009, 21,1434–1449.
25 S. K. Hau, H. L. Yip, H. Ma and A. K. Y. Jen, Appl. Phys. Lett.,2008, 93, 3.
26 K. Sivula, Z. T. Ball, N.Watanabe and J.M. J. Frechet,Adv.Mater.,2006, 18, 206–210.
27 Y. Kim, S. A. Choulis, J. Nelson, D. D. C. Bradley, S. Cook andJ. R. Durrant, Appl. Phys. Lett., 2005, 86, 063502.
28 X. N. Yang, J. Loos, S. C. Veenstra,W. J. H. Verhees, M.M.Wienk,J. M. Kroon, M. A. J. Michels and R. A. J. Janssen, Nano Lett.,2005, 5, 579–583.
29 X. N. Yang, J. K. J. van Duren, R. A. J. Janssen, M. A. J. Michelsand J. Loos, Macromolecules, 2004, 37, 2151–2158.
30 B. J. Kim, Y. Miyamoto, B. W.Ma and J. M. J. Frechet, Adv. Funct.Mater., 2009, 19, 2273–2281.
31 Y. J. Cheng, C. H. Hsieh, Y. J. He, C. S. Hsu and Y. F. Li, J. Am.Chem. Soc., 2010, 132, 17381–17383.
32 Y. J. Cheng, C. H. Hsieh, P. J. Li and C. S. Hsu, Adv. Funct. Mater.,2011, 21, 1723–1732.
33 C. H. Hsieh, Y. J. Cheng, P. J. Li, C. H. Chen, M. Dubosc,R. M. Liang and C. S. Hsu, J. Am. Chem. Soc., 2010, 132, 4887–4893.
34 C. W. Tang and S. A. Vanslyke,Appl. Phys. Lett., 1987, 51, 913–915.35 G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science,
1995, 270, 1789–1791.36 J. E. Slota, X. M. He and W. T. S. Huck, Nano Today, 2010, 5, 231–
242.37 K. H. Lee, P. E. Schwenn, A. R. G. Smith, H. Cavaye, P. E. Shaw,
M. James, K. B. Krueger, I. R. Gentle, P. Meredith and P. L. Burn,Adv. Mater., 2011, 23, 766–770.
38 C. Winder and N. S. Sariciftci, J. Mater. Chem., 2004, 14, 1077–1086.
39 K. M. Coakley, Y. X. Liu, C. Goh and M. D. McGehee,MRS Bull.,2005, 30, 37–40.
40 K. M. Coakley, B. S. Srinivasan, J. M. Ziebarth, C. Goh, Y. X. Liuand M. D. McGehee, Adv. Funct. Mater., 2005, 15, 1927–1932.
41 H. Spanggaard and F. C. Krebs, Sol. EnergyMater. Sol. Cells, 2004,83, 125–146.
42 F. S. Kim, G. Q. Ren and S. A. Jenekhe, Chem. Mater., 2011, 23,682–732.
43 C. Y. Kuo, W. C. Tang, C. Gau, T. F. Guo and D. Z. Jeng, Appl.Phys. Lett., 2008, 93, 3.
44 J. Boucle, S. Chyla, M. S. P. Shaffer, J. R. Durrant, D. D. C. Bradleyand J. Nelson, C. R. Phys., 2008, 9, 110–118.
45 A. M. Peiro, P. Ravirajan, K. Govender, D. S. Boyle, P. O’Brien,D. D. C. Bradley, J. Nelson and J. R. Durrant, J. Mater. Chem.,2006, 16, 2088–2096.
46 T. W. Zeng, I. S. Liu, F. C. Hsu, K. T. Huang, H. C. Liao andW. F. Su, Opt. Express, 2010, 18, A357–A365.
47 H. Xin, F. S. Kim and S. A. Jenekhe, J. Am. Chem. Soc., 2008, 130,5424–5424.
48 H. Xin, G. Q. Ren, F. S. Kim and S. A. Jenekhe, Chem. Mater.,2008, 20, 6199–6207.
49 B. Q. Sun and N. C. Greenham, Phys. Chem. Chem. Phys., 2006, 8,3557–3560.
50 K. J. Ihn, J. Moulton and P. Smith, J. Polym. Sci., Part B: Polym.Phys., 1993, 31, 735–742.
This journal is ª The Royal Society of Chemistry 2011
51 W. D. Oosterbaan, V. Vrindts, S. Berson, S. Guillerez, O. Douheret,B. Ruttens, J. D’Haen, P. Adriaensens, J. Manca, L. Lutsen andD. Vanderzande, J. Mater. Chem., 2009, 19, 5424–5435.
52 K. Vandewal, W. D. Oosterbaan, S. Bertho, V. Vrindts, A. Gadisa,L. Lutsen, D. Vanderzande and J. V.Manca,Appl. Phys. Lett., 2009,95, 123303.
53 S. Berson, R. De Bettignies, S. Bailly and S. Guillerez, Adv. Funct.Mater., 2007, 17, 1377–1384.
54 S. Bertho, W. D. Oosterbaan, V. Vrindts, J. D’Haen, T. J. Cleij,L. Lutsen, J. Manca and D. Vanderzande, Org. Electron., 2009,10, 1248–1251.
55 J. S. Kim, J. H. Lee, J. H. Park, C. Shim, M. Sim and K. Cho, Adv.Funct. Mater., 2011, 21, 480–486.
56 T. Salim, S. Y. Sun, L. H.Wong, L. F. Xi, Y. L. Foo and Y.M. Lam,J. Phys. Chem. C, 2010, 114, 9459–9468.
57 T. G. Jiu, P. Reiss, S. Guillerez, R. de Bettignies, S. Bailly andF. Chandezon, IEEE J. Sel. Top. Quantum Electron., 2010, 16,1619–1626.
58 H. Xin, O. G. Reid, G. Q. Ren, F. S. Kim, D. S. Ginger andS. A. Jenekhe, ACS Nano, 2010, 4, 1861–1872.
59 P. T. Wu, H. Xin, F. S. Kim, G. Q. Ren and S. A. Jenekhe,Macromolecules, 2009, 42, 8817–8826.
60 G. Q. Ren, P. T.Wu and S. A. Jenekhe,ACSNano, 2011, 5, 376–384.61 N. Kiriy, E. Jahne, H. J. Adler, M. Schneider, A. Kiriy,
G. Gorodyska, S. Minko, D. Jehnichen, P. Simon, A. A. Fokinand M. Stamm, Nano Lett., 2003, 3, 707–712.
62 A. J. Moule and K. Meerholz, Adv. Mater., 2008, 20, 240–245.63 L. G. Li, G. H. Lu and X. N. Yang, J. Mater. Chem., 2008, 18, 1984–
1990.64 Y. Zhao, S. Y. Shao, Z. Y. Xie, Y. H. Geng and L. X.Wang, J. Phys.
Chem. C, 2009, 113, 17235–17239.65 J. H. Kim, J. H. Park, J. H. Lee, J. S. Kim, M. Sim, C. Shim and
K. Cho, J. Mater. Chem., 2010, 20, 7398–7405.66 S. Y. Sun, T. Salim, L. H.Wong, Y. L. Foo, F. Boey and Y.M. Lam,
J. Mater. Chem., 2011, 21, 377–386.67 B. G. Kim, M. S. Kim and J. Kim, ACS Nano, 2010, 4, 2160–2166.68 N. Kurokawa, H. Yoshikawa, N. Hirota, K. Hyodo and
H. Masuhara, ChemPhysChem, 2004, 5, 1609–1615.69 C. Szymanski, C. F. Wu, J. Hooper, M. A. Salazar, A. Perdomo,
A. Dukes and J. McNeill, J. Phys. Chem. B, 2005, 109, 8543–8546.70 C. F. Wu, C. Szymanski and J. McNeill, Langmuir, 2006, 22, 2956–
2960.71 H. Kasai, H. S. Nalwa, H. Oikawa, S. Okada, H. Matsuda,
N. Minami, A. Kakuta, K. Ono, A. Mukoh and H. Nakanishi,Jpn. J. Appl. Phys., 1992, 31, L1132–L1134.
72 H. Yabu, T. Higuchi and M. Shimomura, Adv. Mater., 2005, 17,2062–2065.
73 K. Landfester, R. Montenegro, U. Scherf, R. Guntner,U. Asawapirom, S. Patil, D. Neher and T. Kietzke, Adv. Mater.,2002, 14, 651–655.
74 T. Piok, S. Gamerith, C. Gadermaier, H. Plank, F. P. Wenzl, S. Patil,R. Montenegro, T. Kietzke, D. Neher, U. Scherf, K. Landfester andE. J. W. List, Adv. Mater., 2003, 15, 800–804.
75 T. Kietzke, B. Stiller, K. Landfester, R. Montenegro and D. Neher,Synth. Met., 2005, 152, 101–104.
76 T. Kietzke, D. Neher, K. Landfester, R. Montenegro, R. Guntnerand U. Scherf, Nat. Mater., 2003, 2, 408–412.
77 T. Kietzke, D. Neher, M. Kumke, R. Montenegro, K. Landfesterand U. Scherf, Macromolecules, 2004, 37, 4882–4890.
78 H. J. Snaith and R. H. Friend, Synth. Met., 2004, 147, 105–109.79 K. Tada and M. Onoda, Thin Solid Films, 2005, 477, 187–192.80 F. S. Bates and G. H. Fredrickson, Annu. Rev. Phys. Chem., 1990,
41, 525–557.81 C. Park, J. Yoon and E. L. Thomas, Polymer, 2003, 44, 6725–6760.82 T. L. Morkved, M. Lu, A. M. Urbas, E. E. Ehrichs, H. M. Jaeger,
P. Mansky and T. P. Russell, Science, 1996, 273, 931–933.83 T. Thurn-Albrecht, J. Schotter, C. A. Kastle, N. Emley,
T. Shibauchi, L. Krusin-Elbaum, K. Guarini, C. T. Black,M. T. Tuominen and T. P. Russell, Science, 2000, 290, 2126–2129.
84 G. Kim and M. Libera, Macromolecules, 1998, 31, 2569–2577.85 S. B. Darling, Energy Environ. Sci., 2009, 2, 1266–1273.86 S. M. Lindner, S. Huttner, A. Chiche, M. Thelakkat and
G. Krausch, Angew. Chem., Int. Ed., 2006, 45, 3364–3368.87 M. Sommer, S. M. Lindner and M. Thelakkat, Adv. Funct. Mater.,
2007, 17, 1493–1500.
Polym. Chem., 2011, 2, 2707–2722 | 2721
Publ
ishe
d on
24
Aug
ust 2
011.
Dow
nloa
ded
on 2
5/04
/201
4 02
:09:
05.
View Article Online
88 Q. L. Zhang, A. Cirpan, T. P. Russell and T. Emrick,Macromolecules, 2009, 42, 1079–1082.
89 J. U. Lee, A. Cirpan, T. Emrick, T. P. Russell and W. H. Jo, J.Mater. Chem., 2009, 19, 1483–1489.
90 Y. F. Tao, B. McCulloch, S. Kim and R. A. Segalman, Soft Matter,2009, 5, 4219–4230.
91 M. F. Zhang, L. Yang, S. Yurt, M. J. Misner, J. T. Chen,E. B. Coughlin, D. Venkataraman and T. P. Russell, Adv. Mater.,2007, 19, 1571–1576.
92 B. W. Boudouris, C. D. Frisbie and M. A. Hillmyer,Macromolecules, 2008, 41, 67–75.
93 M. C. Iovu, E. E. Sheina, R. R. Gil and R. D. McCullough,Macromolecules, 2005, 38, 8649–8656.
94 M. Jeffries-El, G. Sauve and R. D. McCullough, Macromolecules,2005, 38, 10346–10352.
95 M. C. Iovu, M. Jeffries-El, E. E. Sheina, J. R. Cooper andR. D. McCullough, Polymer, 2005, 46, 8582–8586.
96 C. A. Dai, W. C. Yen, Y. H. Lee, C. C. Ho and W. F. Su, J. Am.Chem. Soc., 2007, 129, 11036–11038.
97 I. Botiz and S. B. Darling, Macromolecules, 2009, 42, 8211–8217.98 I. Botiz, A. B. F. Martinson and S. B. Darling, Langmuir, 2010, 26,
8756–8761.99 J. I. Lee, S. H. Cho, S. M. Park, J. K. Kim, J. W. Yu, Y. C. Kim and
T. P. Russell, Nano Lett., 2008, 8, 2315–2320.100 C. Yang, J. K. Lee, A. J. Heeger and F.Wudl, J.Mater. Chem., 2009,
19, 5416–5423.101 P. Peumans and S. R. Forrest, Appl. Phys. Lett., 2001, 79, 126–128.102 H. Benten, M. Ogawa, H. Ohkita and S. Ito, Adv. Funct. Mater.,
2008, 18, 1563–1572.103 E. W. L. Chan, D. C. Lee, M. K. Ng, G. H. Wu, K. Y. C. Lee and
L. P. Yu, J. Am. Chem. Soc., 2002, 124, 12238–12243.104 Z. Liang, O. M. Cabarcos, D. L. Allara and Q. Wang, Adv. Mater.,
2004, 16, 823–827.105 Z. Q. Liang and Q. Wang, Langmuir, 2004, 20, 9600–9606.106 Z. Q. Liang, K. L. Dzienis, J. Xu and Q. Wang, Adv. Funct. Mater.,
2006, 16, 542–548.107 R. Kniprath, J. T. McLeskey, J. P. Rabe and S. Kirstein, J. Appl.
Phys., 2009, 105, 124313.108 B. Vercelli, G. Zotti, A. Berlin and M. Natali, Chem. Mater., 2010,
22, 2001–2009.109 D. Cheyns, K. Vasseur, C. Rolin, J. Genoe, J. Poortmans and
P. Heremans, Nanotechnology, 2008, 19, 424016.110 M. S. Kim, J. S. Kim, J. C. Cho, M. Shtein, L. J. Guo and J. Kim,
Appl. Phys. Lett., 2007, 90, 123113.111 M. Aryal, F. Buyukserin, K. Mielczarek, X. M. Zhao, J. M. Gao,
A. Zakhidov and W. C. Hu, J. Vac. Sci. Technol., B:Microelectron. Nanometer Struct.–Process., Meas., Phenom., 2008,26, 2562–2566.
112 X. M. He, F. Gao, G. L. Tu, D. G. Hasko, S. Huttner,N. C. Greenham, U. Steiner, R. H. Friend and W. T. S. Huck,Adv. Funct. Mater., 2011, 21, 139–146.
113 M. Aryal, K. Trivedi and W. C. Hu, ACS Nano, 2009, 3, 3085–3090.114 M. Zhou, M. Aryal, K.Mielczarek, A. Zakhidov andW. Hu, J. Vac.
Sci. Technol., B: Microelectron. Nanometer Struct.–Process., Meas.,Phenom., 2010, 28, C6M63–C6M67.
115 Y. Yang, M. Aryal, K. Mielczarek, W. Hu and A. Zakhidov, J. Vac.Sci. Technol., B: Microelectron. Nanometer Struct.–Process., Meas.,Phenom., 2010, 28, C6M104–C6M107.
116 W. Wiedemann, L. Sims, A. Abdellah, A. Exner, R. Meier,K. P. Musselman, J. L. MacManus-Driscoll, P. Muller-Buschbaum, G. Scarpa, P. Lugli and L. Schmidt-Mende, Appl.Phys. Lett., 2010, 96, 263109.
117 S. I. Na, K. Seok-Soon, S. S. Kwon, J. Jang, K. Juhwan, T. Lee andK. Dong-Yu, Appl. Phys. Lett., 2007, 91, 173509.
118 C. F. Shih, K. T. Hung, J. W. Wu, C. Y. Hsiao and W. M. Li, Appl.Phys. Lett., 2009, 94, 143505.
119 X. M. He, F. Gao, G. L. Tu, D. Hasko, S. Huttner, U. Steiner,N. C. Greenham, R. H. Friend and W. T. S. Huck, Nano Lett.,2010, 10, 1302–1307.
120 W. J. Zeng, K. S. L. Chong, H. Y. Low, E. L. Williams, T. L. Tamand A. Sellinger, Thin Solid Films, 2009, 517, 6833–6836.
2722 | Polym. Chem., 2011, 2, 2707–2722
121 M. Steinhart, J. H. Wendorff, A. Greiner, R. B. Wehrspohn,K. Nielsch, J. Schilling, J. Choi and U. Gosele, Science, 2002, 296,1997–1997.
122 M. Steinhart, R. B. Wehrspohn, U. Gosele and J. H. Wendorff,Angew. Chem., Int. Ed., 2004, 43, 1334–1344.
123 M. F. Zhang, P. Dobriyal, J. T. Chen, T. P. Russell, J. Olmo andA. Merry, Nano Lett., 2006, 6, 1075–1079.
124 J. T. Chen, M. F. Zhang and T. P. Russell, Nano Lett., 2007, 7, 183–187.
125 C. R. Martin, Science, 1994, 266, 1961–1966.126 J. T. Chen, D. Chen and T. P. Russell, Langmuir, 2009, 25, 4331–
4335.127 D. Chen, J. T. Chen, E. Glogowski, T. Emrick and T. P. Russell,
Macromol. Rapid Commun., 2009, 30, 377–383.128 J. T. Chen, K. Shin, J. M. Leiston-Belanger, M. F. Zhang and
T. P. Russell, Adv. Funct. Mater., 2006, 16, 1476–1480.129 A. Huczko, Appl. Phys. A: Mater. Sci. Process., 2000, 70, 365–
376.130 P. Apel, Radiat. Meas., 2001, 34, 559–566.131 D. Routkevitch, T. Bigioni, M. Moskovits and J. M. Xu, J. Phys.
Chem., 1996, 100, 14037–14047.132 D. Almawlawi, N. Coombs and M. Moskovits, J. Appl. Phys., 1991,
70, 4421–4425.133 H. Masuda and K. Fukuda, Science, 1995, 268, 1466–
1468.134 A. P. Li, F. Muller, A. Birner, K. Nielsch and U. Gosele, J. Appl.
Phys., 1998, 84, 6023–6026.135 N. Haberkorn, S. A. L. Weber, R. Berger and P. Theato, ACS Appl.
Mater. Interfaces, 2010, 2, 1573–1580.136 M. K. Choi, H. Yoon, K. Lee and K. Shin, Langmuir, 2011, 27,
2132–2137.137 N. Haberkorn, J. S. Gutmann and P. Theato, ACS Nano, 2009, 3,
1415–1422.138 J. Cao, J. Z. Sun, G. Q. Shi, H. Z. Chen, Q. L. Zhang, D. J. Wang
and M. Wang, Mater. Chem. Phys., 2003, 82, 44–48.139 D. Huesmann, P. M. DiCarmine and D. S. Seferos, J. Mater. Chem.,
2011, 21, 408–413.140 H. S. Wang, L. H. Lin, S. Y. Chen, Y. L. Wang and K. H. Wei,
Nanotechnology, 2009, 20, 075201.141 H. S. Wang, S. Y. Chen, M. H. Su, Y. L. Wang and K. H. Wei,
Nanotechnology, 2010, 21, 145203.142 J. S. Kim, Y. Park, D. Y. Lee, J. H. Lee, J. H. Park, J. K. Kim and
K. Cho, Adv. Funct. Mater., 2010, 20, 540–545.143 J. H. Jeon, J. H. Keum, J. H. Song and J. S. Kim, Sol. EnergyMater.
Sol. Cells, 2007, 91, 645–651.144 A. F. Nogueira, C. Longo and M. A. De Paoli, Coord. Chem. Rev.,
2004, 248, 1455–1468.145 P. C. Yu, C. H. Chang, M. S. Su, M. H. Hsu and K. H. Wei, Appl.
Phys. Lett., 2010, 96, 153307.146 M. H. Hsu, P. C. Yu, J. H. Huang, C. H. Chang, C. W. Wu,
Y. C. Cheng and C. W. Chu, Appl. Phys. Lett., 2011, 98, 073308.147 D. A. Rider, R. T. Tucker, B. J. Worfolk, K. M. Krause, A. Lalany,
M. J. Brett, J. M. Buriak and K. D. Harris, Nanotechnology, 2011,22, 085706.
148 M. K. Fung, Y. C. Sun, A. Ng, A. M. C. Ng, A. B. Djurisic,H. T. Chan and W. K. Chan, ACS Appl. Mater. Interfaces, 2011,3, 522–527.
149 K. M. Coakley, Y. X. Liu, M. D. McGehee, K. L. Frindell andG. D. Stucky, Adv. Funct. Mater., 2003, 13, 301–306.
150 K. M. Coakley and M. D. McGehee, Appl. Phys. Lett., 2003, 83,3380–3382.
151 H. Wang, C. C. Oey, A. B. Djurisic, M. H. Xie, Y. H. Leung,K. K. Y. Man, W. K. Chan, A. Pandey, J. M. Nunzi andP. C. Chui, Appl. Phys. Lett., 2005, 87, 023507.
152 C. C. Oey, A. B. Djurisic, H. Wang, K. K. Y. Man, W. K. Chan,M. H. Xie, Y. H. Leung, A. Pandey, J. M. Nunzi and P. C. Chui,Nanotechnology, 2006, 17, 706–713.
153 S. S. Kim, J. Jo, C. Chun, J. C. Hong and D. Y. Kim, J. Photochem.Photobiol., A, 2007, 188, 364–370.
154 H. J. Her, J. M. Kim, C. J. Kang and Y. S. Kim, J. Phys. Chem.Solids, 2008, 69, 1301–1304.
This journal is ª The Royal Society of Chemistry 2011